Method for CVD process control for enhancing device performance

ABSTRACT

A method and system for controlling the introduction of a species according to a determined concentration profile of a film comprising the species introduced on a substrate. In one aspect, the method comprises controlling the flow rate of a species according to a determined concentration profile of a film introduced on a substrate, and introducing a film on a substrate, the film comprising the species at a first concentration at a first point in the film and a second concentration different than the first concentration at a second point in the film. Also, a bipolar transistor including a collector layer of a first conductivity type, a base layer of a second conductivity type forming a first junction with the collector layer, and an emitter layer of the first conductivity type forming a second junction with the base layer. An electrode configured to direct carriers through the emitter layer to the base layer and into the collector layer is also included. In one embodiment, at least one of the first junction and the second junction is between different semiconductor materials to form at least one heterojunction. The heterojunction has a concentration profile of a semiconductor material such that an electric field changes in an opposite way to that of a mobility change.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to semiconductor processing techniques,more particularly, to controlling constituents of a film introduced ontoa substrate.

[0003] 2. Description of Related Art

[0004] In the formation of modern integrated circuit devices, manyconstituents are introduced to a substrate such as a wafer to formfilms. Typical films include dielectric material films, such astransistor gate oxide or interconnect isolation films, as well asconductive material or semiconducting material films. Interconnect metalfilms and polysilicon electrode films, respectively, are examples ofconductive and semiconducting material films.

[0005] In addition to the above-noted material films, other constituentsare often introduced onto a substrate such as a wafer or a structure ona substrate to change the chemical or conductive properties of thesubstrate or the structure. Examples of this type of constituentintroduction includes, for example, the deposition of a refractory metalonto an electrode or junction to form a silicide and the deposition ofgermane onto a substrate to form a silicon germanium junction in abipolar transistor. The introduction of constituents onto a substrate orstructure on a substrate such as described is referred to herein as asubset of film formation.

[0006] One way to enhance the performance of integrated circuit devicesis to improve control of the introduction of the constituents, such asimproved control of the introduction of process gas species indeposition introduction. Many wafer process chambers, including the EPICentura system, commercially available from Applied Materials, Inc. ofSunnyvale, Calif., utilize mass flow controllers to introduce processgas species. In general, a mass flow controller functions by permittinga desired flow rate of a gas species based on an input signal to themass flow controller demanding the Flow rate. The concentration profileof a species constituent within a film deposited on a substrate is thena function of the mass flow rate of species introduced. In general, therelationship between a species concentration profile or gradientintroduced into or onto a substrate, for example a wafer, and the massflow rate of the species introduced is not necessarily linear.

[0007] In general, mass flow controllers are used to either supply aconstant flow rate or a variable flow rate from a first flow set pointto a second flow set point over a period of time. One common flow rampbetween a first set point and a second set point is a linear flow ramp.A linear ramp, however, does not necessarily produce a desiredconcentration profile, e.g., a linear profile, of the species in theintroduced film. In the example of a species of germane (GeH₄)introduced to form a silicon-germanium film, a graded film is desirablein many situations. The desired graded profile in the film, for examplegermanium concentration profile, may be linear or non-linear. The methodto control a mass flow controller to precisely control the amount offlow and produce the desired germanium concentration profile in thejunction, whether it is linear or non-linear, is of significantimportance. In commercial use, targeting a desired concentrationprofile, for example a linear profile, has generally not proved possiblethrough a linearly increasing or decreasing constant flow introductionof the germanium species by a mass flow controller.

[0008] What is needed is a way to control the introduction of a speciesto form a film having a desired concentration profile of the species inthe film. The ability to quantitatively control the introduction of aspecies through a mass flow controller to form a film with a specificfilm thickness is also desirable.

SUMMARY OF THE INVENTION

[0009] A method and system for controlling the introduction of a speciesof a film comprising the species introduced on a substrate is disclosed.In one aspect, the method comprises controlling the flow rate of aspecies according to a determined graded concentration profile of a filmintroduced on a substrate, and introducing a film on a substrate, thefilm comprising the species at a first concentration at a first point inthe growth of the film and a second concentration different than thefirst concentration at a second point in the growth of the film. In oneembodiment, the concentration profile used to control the flow rate isestablished by experimentally determining a concentration of the speciesintroduced on a substrate for a first plurality of flow rates anddetermining an introduction rate, e.g., a growth rate, of the speciesintroduced, e.g., grown on a substrate. According to the invention, moreaccurate control of a species concentration in a formed film can beobtained over prior art methods. The invention also offers the abilityto control the amount or the thickness of a film formed on a substrate.

[0010] A bipolar transistor is also disclosed. In one embodiment, thebipolar transistor includes a collector layer of a first conductivitytype, a base layer of a second conductivity type forming a firstjunction with the collector layer, and an emitter layer of the firstconductivity type forming a second junction with the base layer. Anelectrode configured to direct carriers through the emitter layer to thebase layer and into the collector layer is also included. In thisembodiment, at least one of the first junction and the second junctionis between different semiconductor materials to form at least oneheterojunction. The heterojunction has a concentration profile of asemiconductor material such that an electric field changes in anopposite way to that of a mobility change.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 schematically illustrates a side view of a portion of asubstrate having a film with a graded concentration profile introducedaccording to an embodiment of the invention.

[0012]FIG. 2 illustrates the concentration gradient of a germaniumspecies in a film according to an embodiment of the invention.

[0013]FIG. 3 illustrates a schematic view of an embodiment of a systemfor introducing a species to a substrate according to the invention.

[0014]FIG. 4 illustrates a curve fit of the experimentally-determinedconcentration of germanium of a film introduced according to sixdiscrete germane flow rates and a constant silane flow rate.

[0015]FIG. 5 illustrates the experimentally-determined growth rate ofsilicon germanium in a film introduced on a substrate for six discretegermane flow rates and a constant silane flow rate.

[0016]FIG. 6 illustrates a block diagram for the introduction of agermanium species to a substrate in accordance with an embodiment of theinvention.

[0017]FIG. 7 is a Secondary Ion Mass Spectroscopy (SIMS) profile of anepitaxial silicon-germanium film introduced according to the inventionto have a linearly graded profile of germanium.

[0018]FIG. 8 is the flow rate of germane (GeH₄) per unit time to producethe graded profile of FIG. 7.

[0019]FIG. 9 is a SIMS profile of an epitaxial silicon-germanium filmintroduced according to the invention to have a concave graded profileof germanium.

[0020]FIG. 10 is the flow rate of GeH₄ per unit time to produce thegraded profile of FIG. 9.

[0021]FIG. 11 schematically illustrates a heterojunction bipolartransistor formed according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] A method and a system for the controlled introduction of aspecies to a substrate are disclosed. In one embodiment, the methodincludes controlling the flow rate of a species into a chamber accordingto determined concentration and introduction rate profiles to introduce(e.g., deposit) a film on a substrate in the chamber. The determinedconcentration and introduction rate profiles may be established throughexperimental data related to a concentration of a species in a formedfilm according to a plurality of selected flow rates of the speciesconstituent (hereinafter “species”) into the chamber. This informationis utilized to adjust the introduction rate of a species per unit timeto form a film having a desired concentration profile as well as adesired thickness.

[0023]FIG. 1 shows a side view of a portion of a semiconductor substratehaving a silicon germanium (Si_(1-x)Ge_(x)) epitaxially-introduced filmthereon. Structure 10 includes substrate 25 that is, for example, asilicon semiconductor wafer with Si_(1-x)Ge_(x) film 30 introduced on asurface thereof. Si_(1-x)Ge_(x) film 30, in this embodiment, has agraded concentration profile of germanium (Ge), represented byconcentration points 35 and 40. Concentration points 35 and 40represent, for example, two of many concentration points. In oneexample, the concentration profile from concentration point 35 toconcentration point 40 is desired to be linear with the highestconcentration of Ge present at concentration point 40 and the lowestconcentration at concentration point 35. In one example, theconcentration profile of Ge in Si_(1-x)Ge_(x) film 30 varies linearlyfrom a concentration of approximately zero percent Ge at concentrationpoint 35 to a concentration of 20 percent Ge at concentration point 40.

[0024]FIG. 2 graphically represents the concentration profile ofgermanium in Si_(1-x)Ge_(x) film 30 of FIG. 1. In this representation,the concentration profile is measured from the surface of the film(represented by concentration point 35) to the silicon-Si_(1-x)Ge_(x)film interface (represented by concentration point 40). Thus, the filmthickness is measured from the surface of film 30 to the interface offilm 30 and substrate 25. In one example, denoted by the solid line, theconcentration profile varies in a linear fashion through the film. It isto be appreciated that the invention method and system is capable ofproducing a variety of concentration profiles, including non-linearprofiles such as profile 210 and profile 220 in FIG. 2.

[0025]FIG. 3 is an example of a process environment utilizing a systemof the invention to introduce a species to form a film such as film 30on substrate 25 of FIG. 1. In this embodiment, an EPI Centura system,commercially available from Applied Materials, Inc. of Sunnyvale,Calif., modified according to the invention is described. It is to beappreciated that the system is not limited to an EPI Centura system butcan be accommodated in other systems, particularly where a mass flowcontroller is utilized to introduce a species into a reaction chamber. ASi_(1-x)Ge_(x) chemical vapor deposition (CVD) film formation process isalso described. Similarly, it is to be appreciated that the invention isnot limited to CVD Si_(1-x)Ge_(x) film formation systems but will applyto other systems and methods, particularly where a species is introducedinto a chamber to form a film.

[0026] Referring to FIG. 3, the system includes chamber 100 thataccommodates substrate 25, such as a semiconductor wafer, forprocessing. Substrate 25 is seated on stage 125 that is, in oneembodiment, a susceptor plate. Heating lamps 102 and 104 are used toheat substrate 25. Processor 110 controls the temperature and pressureinside chamber 100. The temperature is measured via, for example,pyrometers 106 and 108 coupled to the chamber. Similarly, the pressuremay be monitored by one or more pressure sensors, such as BARATRON®pressure sensors, commercially available from MKS Instruments ofAndover, Massachusetts, and regulated by a pressure control value. Inthe schematic illustration shown in FIG. 3, pyrometer 106 and pyrometer108 are coupled to processor 110 through signal line 115. Processor 110uses received information about the substrate temperature to controlheat lamps 102 and 104. The one or more pressure sensors are coupled toprocessor 110 through signal line 135. Processor 110 uses receivedinformation about the chamber pressure to control the pressure through,for example, controlling a vacuum source and a pressure control valuecoupled to the chamber.

[0027] Processor 110 also controls the entry of constituents intochamber 100. In one embodiment, the system includes at least two sourcegases 120 and 130 coupled to manifold 105. Processor 110 controls theintroduction of each of source gas 120 and source gas 130, as desired,through manifold 105 and controls the flow of the source gas or gasesthrough mass flow controllers 160 and 162, respectively. For use in aSi_(1-x)Ge_(x) film formation process, mass flow controller 160 is, forexample, a one standard liter per minute (SLM) of silane (SiH₄) unit andmass flow controller 162 is a 150 standard cubic centimeters per minute(sccm) of germane (GeH₄) unit. Processor 110 also controls theintroduction of a process gas (source gas 180), such as for example,nitrogen (N₂) or hydrogen (H₂), through mass flow controller 168 asknown in the art. Each mass flow controller is, for example, a unitcommercially available from UNIT Instruments, Inc. of Yorba Linda,Calif.

[0028] In one embodiment, processor 110 controls the introduction of asource gas to form a Si_(1-x)Ge_(x) film on substrate 25, such asSi_(1-x)Ge_(x) film 30 in FIG. 1. Source gas 120 is, for example, theconstituent silane (SiH₄) and source gas 130 is, for example, theconstituent germane (GeH₄). In this embodiment, one goal is to introducea film having a concentration gradient of the species germanium (Ge)through the thickness of the Si_(1-x)Ge_(x) film. Still further, thisgradient is desired, in one embodiment, to be linear between aconcentration of Ge at a surface of the film (concentration point 35 ofFIG. 1) of zero percent and a maximum at an interface between thesilicon wafer and the film (concentration point 40 of FIG. 1).

[0029] In general, mass flow controllers, such as mass flow controller160 and mass flow controller 162, can vary (e.g., increase or decrease)the flow rate change of a species introduced into a chamber. Theconcentration change of a species such as Ge over a thickness of a filmmay be accomplished at the mass flow controller by changing the flowrate of the source gas into chamber 100. For any measurable control,this flow rate change is generally linear. However, a linear flow ratechange, for example, from higher to lower mass flow and thus lowerintroduction amount of species, does not necessarily produce a linearconcentration gradient of the species in the formed film. This isparticularly the case with the constituent GeH₄, where a linear increaseor decrease of flow rate does not generally result in a linear change inconcentration of the species constituent Ge in the formed film.

[0030] Instead, a concentration profile of Ge in a formed film generallymore closely resembles the convex profile represented by dashed line210. According to the invention, however, a concentration profile suchas represented by line 200 may be obtained by controlling mass flowcontroller 162 to introduce source gas 130 at a non-linear rate.

[0031] In an embodiment of the invention, a method is presented whereina desired concentration profile of the species, including the linearconcentration profile illustrated in FIG. 2 (line 200), is produced.According to this method, experimental determinations of theconcentration of a species such as Ge is measured in a film formedaccording to a plurality of flow rates of the constituent GeH₄ throughmass flow controller 162 on a sacrificial wafer. In one embodiment, theconcentration of Ge in a film formed on a wafer is measured for sixdiscrete flow rates of GeH₄ through mass flow controller 162. Eachexperimental measurement corresponds to a single unit of film introducedon a wafer by introducing a constant flow rate of GeH₄ through mass flowcontroller 162. In one embodiment, six concentrations of Ge in sixdiscrete films introduced by six discrete GeH₄ flow rates through massflow controller 162 on six wafers are measured. Each film is analyzedfor species Ge concentration through analytical methods such asSecondary Ion Mass Spectroscopy (SIMS), x-ray diffraction, orellipsometry. The six discrete flow rates are plotted versus the Geconcentration in a corresponding film as illustrated in FIG. 4. In thisexample, a single experimental measurement is obtained from a film on asingle wafer by placing a sacrificial wafer in chamber 100 and thereaction conditions of the chamber established. In one embodiment, afilm is formed at a chamber pressure of 100 Torr and a temperature of680° C. In one example, process gas is introduced in chamber 100according to the following flow rate recipe: Source Gas Constituent FlowRate 120 SiH₄  1 SLM 130 GeH₄ varied 180 H₂ 30 SLM

[0032] It is to be appreciated, that other recipes may be utilized tointroduce the films on the wafers. Such recipes will generally depend onthe desired process parameters. For example, in the introduction of aSi_(1-x)Ge_(x) film, additional constituents such as hydrochloric acid(HCl), may be added to modify the properties of the film. One objectivein collecting the experimental data is to mimic the desired processconditions as closely as possible.

[0033] According to the above recipe, six flow rates of source gas 130of the constituent GeH₄ are selected between 0 and 300 sccm. It is to beappreciated that GeH₄ flow rates higher than 300 sccm can be selected.One limit of GeH₄ flow may be considered as one beyond which the Geconcentration in the introduced film will not further increase for anincrease in the GeH₄ flow rate. A corresponding concentration of Ge ismeasured in a film formed on the sacrificial wafer. Once the data iscollected, a curve is established through a curve fit algorithm such asa Gauss-Jurdan algorithm. FIG. 4 illustrates the curve fit for sixpoints. In one example, a Gauss-Jurdan numerical algorithm is used tocalculate the coefficients of a third order polynomial that best fitsthe six experimental measurements. This method of curve fitting is knownas the Least Square Fit (LSF) method of curve fit. It is to beappreciated that the Gauss-Jurdan numerical algorithm is not the onlymethod to calculate coefficients of an LSF polynomial. Similarly, theLSF method as well as the Gauss-Jurdan method are not limited to sixdata points but may be used, for example, with as few as three datapoints or more than six data points.

[0034] According to an embodiment of the invention, the flow rate ofGeH₄ (e.g., six discrete flow rate measurements) is also measuredagainst the introduction rate, e.g., growth rate, of the Si_(1-x)Ge_(x)film introduced on the sacrificial wafer. It is to be appreciated thatthe same six flow rates as utilized in FIG. 4 may be utilized to comparethe growth rate of Si_(1-x)Ge_(x) In one embodiment, theexperimentally-obtained Si_(1-x)Ge_(x) growth rates are measured fromthe same film grown on the same six sacrificial wafers used to measureGe concentration. FIG. 5 shows a plot of GeH₄ flow rate versusSi_(1-x)Ge_(x) growth rate and a curve fit through the plotted points.The same numerical method of LSF is used to determine the best curve fitto the experimentally-obtained growth rate measurements. Thecoefficients of the third order polynomial are calculated using, forexample, the Gauss-Jurdan method noted above with respect to FIG. 4.

[0035] The experimentally-determined data for concentration of a speciesas a function of flow rate and the experimentally-determined data forgrowth rate as a function of flow rate is input into processor 110.Also, a desired Ge concentration profile as a function of theSi_(1-x)Ge_(x) film thickness is input into processor 110. For example,Si_(1-x)Ge_(x) film 30 in FIG. 1 is formed for input germaniumconcentrations of 20% at point 40 and 0% at point 35 having a linearchange in concentration from point 40 to point 35 over a 500 AngstromsSi_(1-x)Ge_(x) input film thickness identified as film 30.

[0036] When wafer 10 is placed in processor 110, processor 110 firstcalculates the curves of FIGS. 4 and 5 using the six experimentallydetermined measures for concentration and growth rate. Processor 110next uses the desired input concentration profile over the desiredgrowth thickness as a guide to calculate the set points for GeH₄ massflow controller 162. For a desired Ge concentration, the correspondingGeH₄ flow rate is calculated from FIG. 4. This flow rate is then used tocalculate the Si_(1-x)Ge_(x) growth rate, from FIG. 5. The correspondinggrowth rate used along with a selected time interval (Δt) establishesthe desired growth thickness of a portion of Si_(1-x)Ge_(x) film for thetime interval. The thickness of the Si_(1-x)Ge_(x) grown within aselected time interval is subtracted from the total desired filmthickness to establish the thickness left to be grown. Using the newthickness that yet needs to be grown, the desired input concentrationprofile as a function of thickness is used to calculate a correspondingnew Si_(1-x)Ge_(x) concentration value. Using the new concentrationvalue, the above process of using the data from FIGS. 4 and 5 will berepeated to calculate a new thickness of Si_(1-x)Ge_(x) grown for asecond time interval, Δt. This iterative process will continue until thetotal desired thickness of Si_(1-x)Ge_(x) is on wafer 10.

[0037] In one example, a Si_(1-x)Ge_(x) film has a Ge concentration of20 percent at the wafer film interface (concentration point 40) and zeroat the film surface (concentration point 35). In this example, a linearconcentration profile is desired. Given the desired concentration (e.g.,20 percent), the data obtained from FIG. 4 is queried to obtain thedesired flow rate of GeH₄ species through mass flow controller 162 (flowas a function of concentration). Once the flow is established, the datacollected and represented by FIG. 5 is utilized to calculate a growthrate for the desired flow (growth rate as a function of flow rate). Fora predetermined time interval (e.g., 0.2 seconds), the amount of film 30introduced on substrate 25 during a time interval of 0.2 seconds may bedetermined for the desired concentration. Thus, by using theexperimental data and a predetermined time interval, the concentrationin a Si_(1-x)Ge_(x) film and a film thickness is known.

[0038] In the example where a linear variance in concentration isdesired, such as illustrated by line 200 in FIG. 2, a correspondingconcentration is determined for a second time interval. Thus, inreference to FIG. 2, knowing the concentration profile as a function offilm thickness and starting from a desired concentration point of film30, subsequent concentration points along the path of line 200 may becalculated for time intervals, Δt. In one embodiment, a time interval of0.2 seconds is used to control mass flow controller 162 and thecorresponding GeH₄ flow rate (FIG. 4) as well as growth rate (FIG. 5) iscalculated to obtain a linear profile (line 200). The process continuesuntil a desired film thickness of film 30 is formed.

[0039] Processor 110 contains, in one embodiment, a suitable algorithmto calculate the desired flow rate of a constituent to mass flowcontroller 162 as a function of concentration. Processor 110 alsocontains, in this embodiment, a suitable algorithm to calculate a growthrate as a function of flow rate. For example, processor 110 is suppliedwith software instruction logic that is a computer program stored in acomputer-readable medium such as memory in processor 110. The memory is,for example, a hard disk drive. Additional memory associated withprocessor 110 stores, among another items, the experimentally determineddata of concentration of the constituent species over a desired flowrate spectrum (FIG. 4), and experimentally-determined data related tothe growth rate of the constituent species over the desired flow ratespectrum (FIG. 5) as well as the corresponding curve fit algorithms.

[0040]FIG. 6 shows an illustrative block diagram of the hierarchicalstructure of system logic according to one embodiment of the inventionfor forming a film having a desired concentration profile and thicknesson a substrate which is a wafer. Such control logic would constitute aprogram to be run on processor 110. A suitable programming language forsuch a program includes, but is not limited to, C, C⁺⁻, and otherlanguages. The program may be supplied directly on processor 110 or toprocessor 110 by way of an outside device, such as a computer.

[0041] As a first operation, certain user inputs are supplied toprocessor 110 and stored in the form of either internal or externalmemory. The information supplied to processor 110 for the system logicincludes experimental data for introduction rate, e.g., concentration ofa species such as Ge concentration as a function of mass flow rate(block 310), experimental data for growth rate of an introduced film,such as Si_(1-x)Ge_(x) as a function of mass flow rate (block 320), thedesired thickness of a film on a substrate, such as a Si_(1-x)Ge_(x)film on a wafer (block 330), and the desired concentration profile of afilm formed on a substrate such as a wafer (block 340).

[0042] Once the above-described data is supplied to processor 110, thesystem logic calculates a flow rate of a species, such as GeH₄, for avalue of concentration desired by the user (block 350) for apredetermined time interval. System logic is then used to control massflow controller 162 to regulate the corresponding flow rate of sourcegas 130 of GeH₄.

[0043] In addition to calculating a corresponding flow rate of a speciesfor a desired concentration, the system logic calculates an introductionrate, e.g., a growth rate, of a corresponding film on a substrate, suchas a wafer for the calculated flow rate (block 360). For a calculatedgrowth rate of film, an amount of film can further be calculated for agiven time interval (block 370). This information is used by processor110 to introduce a constituent, such as GeH₄, through mass flowcontroller 162 to introduce a film on substrate 25 for a selected timeinterval. 0.2 seconds is an example of a desired time interval as 0.2seconds represents the time interval utilized for ramp-up or ramp-downof flow through a mass flow controller, for example, a UNIT mass flowcontroller used in an EPI Centura system.

[0044] Once a constituent is introduced into chamber 100 to form aportion of film 30 according to the method described herein for apredetermined time interval, the system logic determines whether thedesired film thickness is achieved by comparing the calculated filmthickness with the desired film thickness (block 380). If the desiredinput film thickness with the desired input concentration profile hasnot been achieved, the system logic of processor 110 calculates a newvalue for film thickness representing the additional thickness amountneeded to obtain the total desired input thickness (block 390), andcalculates the corresponding desired input concentration value for thenewly calculated thickness (block 395). Processor 110 returns to block350 and uses the newly calculated value of desired concentration tocalculate a corresponding flow rate. Processor 110 continues thisprocess until a desired film thickness is achieved. Once a desired filmthickness is achieved, the system logic discontinues the loop andcompletes the film formation (block 396). It is to be appreciated thatcalculations of flow rates and film growth may precede the introductionof a constituent into the chamber.

[0045] According to the method and system described, a film-formingconstituent can be controlled, through control of a mass flowcontroller, to achieve a desired concentration profile of a species in afilm and a desired film thickness introduced on a substrate, such as awafer. In the above embodiment, a method of achieving a linearconcentration profile is described. FIG. 7 shows a SIMS profile of aepitaxially grown Si_(1-x)Ge_(x) film on a silicon substrate having alinear concentration profile of Ge introduced according to a method ofthe invention. The SIMS profile illustrates the atomic profile of Gefrom the surface (0 depth) to the interface of the Si_(1-x)Ge_(x) andthe silicon substrate. Thus, the depth represents the depth into theSi_(1-x)Ge_(x) film.

[0046] In FIG. 7, the thickness of the Si_(1-x)Ge_(x) film isapproximately 1000Å. The concentration at the surface of the film iszero (represented as beginning at a depth of approximately 500Å toaccount for a cap on the SIMS system). The concentration profile throughthe film is linear to a Ge concentration of 16 percent at theSi_(1-x)Ge_(x) /silicon interface.

[0047]FIG. 8 shows a plot of the flow rate of GeH₄ introduced to producethe linear Ge profile illustrated in FIG. 7. FIG. 8 illustrates that alinear concentration profile is formed but the flow rate of GeH₄ thatproduced the profile is varied in a non-linear fashion.

[0048] It is to be appreciated that the principles of the invention arenot limited to a method and system for introducing a film having alinear concentration profile of a species constituent, but are equallyapplicable to situations where a non-linear concentration profile isdesired. For example, a profile such as illustrated by lines 210 and 220in FIG. 2 or other profile may be desired. In one aspect, the inventionprovides a technique for controlling a mass flow controller to achieve adesired concentration profile in a film introduced on a substrate.

[0049]FIG. 9 shows a film profile of epitaxially grown silicon-germaniumfilm on a silicon substrate having a concave concentration profile of Geformed according to a method of the invention. Similar to FIG. 7, thefilm's profile illustrates the atomic profile of Ge from the surface(zero depth) to the interface of the Si_(1-x)Ge_(x) and the siliconsubstrate. The thickness of the silicon-germanium film is approximately1000Å. The concentration profile adapts a concave representation from aGe concentration of zero percent at the surface of the film to aconcentration of 15 percent at the interface.

[0050]FIG. 10 shows a plot of the flowchart of GeH₄ introduced toproduce the profile illustrated in FIG. 9. FIG. 10 illustrates that theconcave profile produced in FIG. 9 is not the result of a linear changein the flow rate of GeH₄.

[0051] In addition to providing the ability to establish a desiredconcentration profile of a film introduced on a substrate, such as awafer, the invention offers a method and system for defining thethickness of a film introduced on a wafer. Accordingly, the inventionmay be practiced so as to achieve a desired concentration profile of aspecies, introduced by mass flow meter, having a desired concentrationprofile and a desired film thickness.

[0052] One application of controlling the introduction of constituentsonto a semiconductor substrate to yield a graded film is in theformation of heterojunction bipolar transistors (HBTs). Bipolartransistors are utilized in a variety of applications including asamplifying and switching devices. HBTs generally offer improvedperformance over traditional bipolar transistors and metal oxidesemiconductor (MOS) transistors in high frequency applications,particularly applications approaching 50 gigahertz (gHz). As higherfrequency applications (e.g., 50 gHz or greater) become desirable, aneed for improved HBTs exist. The invention contemplates improvedperformance of HBTs by utilizing Si_(1-x)Ge_(x) graded junctions havingoptimized concentration profiles.

[0053]FIG. 11 shows a representative example of an HBT according to theinvention. HBT 400 includes emitter region 410, base region 420 andcollector region 430. Emitter-base (E-B) spacer 435 is positionedbetween emitter region 410 and base region 420. Base-collection (B-C)spacer 445 is positioned between base region 420 and collector region430. HBT 400 is characterized by base region 420 of an epitaxiallyformed Si_(1-x)Ge_(x) film as are E-B spacer 435 and B-C spacer 445. Thebipolar transistor, in this embodiment, comprises N-type emitter region410, P-type base region 420, and N-type collector region 430 (a NPNtransistor). FIG. 11 illustrates the movement of electrons throughtransistor 400 in response to a voltage applied through electrode 450.In one example that follows, at least E-B spacer 435 will be formed withconcentration gradient analagous to that shown in FIG. 9 (i.e.,profile).

[0054] The use of Si_(1-x)Ge_(x) in an HBT generally enableshigh-frequency performance. One of the major advantages ofSi_(1-x)Ge_(x) is a smaller energy gap than that of silicon. Inunstrained bulk Si_(1-x)Ge_(x) the energy gap drops from approximately1.1 electron-volts (eV) in silicon to 1.0 eV for Si_(0.8)Ge_(0.2). Thelattice constant of Si_(1-x)Ge_(x) is also larger than the latticeconstant in silicon. If the thickness of the Si_(1-x)Ge_(x) alloy isbelow a critical value, the mismatch in lattice constant is accommodatedelastically, no dislocations are formed, and the Si_(1-x)Ge_(x) film isstrained. Strain lifts the degeneracy of both valence and conductionbands. As a result, the energy gap of strained Si_(1-x)Ge_(x) decreaseseven more than unstrained Si_(1-x)Ge_(x),to approximately 0.9 eV forstrained Si_(0.8)Ge_(0.2).

[0055] The change in the energy gap in strained Si_(1-x)Ge_(x) film, Δ,allows for a fast transmit of charge carriers in the base region (e.g.,base region 420) of HBTs under the action of the drift electric field,E:

E=−dΔ/d1,  (1)

[0056] where 1 is the distance across the base region. For example, theband gap reduction of 0.2 eV across a 500Å base region, translates to adrift electric field of 40 kV/cm.

[0057] Charge carrier drift mobility, μ, through the base region of anHBT is proportional to the scattering time, τ, and inverselyproportional to the effective mass of the carrier, m*:

μ˜τ/m*.  (2)

[0058] The scattering time diminishes with increasing Ge concentrationbecause of alloy scattering. The effective mass, m*, becomes anisotropicbetween “in-plane” and “perpendicular to the junction” directions ofmotion because of strain. For the perpendicular direction, the effectivemass of holes is significantly smaller than Si_(1-x)Ge_(x) due tovalence band offset. The resulting hole mobility augments with theincrease of x, from 450 cm²/Vs for x=0 to 1000 cm²/Vs for x=0.2 inSi_(1-x)Ge_(x) with low dopant (i.e., P-type, N-type) concentration. Theopposite trend takes place for electrons: their perpendicular mobilitydrops from 14000 cm²/Vs for x=0 to 750 cm²/Vs for x=0.2. Despite thedecrease in the electron mobility with increasing x, the electronmobility is larger than the hole mobility over the majority of thepractically useful range of the Ge concentration (i.e., x between 0 and0.2). Accordingly, the transistor configuration of choice for high-speedapplications is generally the NPN transistor because the speciestravelling from the emitter region to the collector region areelectrons.

[0059] Typical P-type doping levels for Si_(1-x)Ge_(x) base region 420of NPN transistor 400 are in the 10¹⁸ cm⁻³-10¹⁹ cm⁻³ range. In oneaspect, this choice is generally determined by the requirement of havingfairly low sheet (i.e., in-plane) resistance of base region 420. Whenthe dopant concentration becomes large, two effects are generallythought to occur: first, the carrier perpendicular mobility issignificantly reduced. For instance, for electrons, the perpendicularmobility is approximately 250 cm²/Vs and 120 cm²/Vs for doping levels of10¹⁸ cm⁻³ and 10¹⁹ cm⁻³, respectively. Second, the mobility dependenceon the Ge concentration is reduced: carrier mobility, μ, is almostx-independent for doping levels in the 10¹⁸ cm⁻³-10¹⁹ cm⁻³ range. Theeffects of dopant concentration is presented in detail in the treatiseSemiconductors and Semimetals, Vol. 56, “Germanium Silicon: Physics andMaterials,” edited by R. Hull and J. C. Bean, in the article by S. A.Ringel and P. N. Grillot, “Electronic Properties and Deep Levels inGe-Si,” at pages 293-346 (1999). The significant reduction in carriermobility can be counterweighted by reducing the perpendicular size ofthe base (to 200Å-300Å or thinner) and by the drift electric field(i.e., built-in potential) (see Equation 1).

[0060] The small size of base region 420 generally results in degradedoverall performance of devices because of low leakage currents andreduced breakdown voltages. In order to overcome this problem, it hasbeen suggested to use lightly doped spacers at the emitter-base andcollector-base regions, e.g., E-B spacer 435 and C-B spacer 445. SeeMeyerson, B. S., et al., “Silicon: Germanium Heterojunction BipolarTransistors; from Experiment to Technology,” Selected Topics inElectronics and Systems, Vol. 2, “Current Trends in HeterojunctionBipolar Transistors,” edited by M. F. Chang. E-B spacer 435 and C-Bspacer 445 may be intrinsic (i.e., no doping) or lightly N-type doped(e.g., typically below 10¹⁷cm⁻³). As for lightly doped Si_(1-x)Ge_(x)the electron perpendicular mobility generally depends on the Geconcentration.

[0061] If the band energy gap changes linearly, the built-in electricfield is constant across the junction (Equation 1). This means thatpassing through the spacers, electrons spend significantly more time inthe high Ge concentration regions (where their mobility is lower) thanthey do in the regions where the Ge concentration is lower. Theinvention recognizes that the overall transient time can besubstantially reduced by creating a Ge concentration profile in such away that the electric field would be higher in the regions wheremobility is lower. In other words, the electric field changes in anopposite way to that of the mobility change (i.e., the electric fieldincreases through the heterojunction if carrier mobility decreases andvice versa) to enhance the cut-off frequency of the transistor.

[0062] In one example, the transient time, τ, through E-B spacer 435 isdetermined for the following two profiles of Ge concentration: Profile Ais a linear grade, and Profile B where the electric field is inverselyproportional to the electron perpendicular mobility to yield a concavegradient. As a simplification, a linear relationship between theelectron drift velocity, v, and the electric field is assumed:

v−μE.  (3)

[0063] In this example, E-B spacer 435 has a thickness, W, in which theGe concentration changes from 0 to x₁. The corresponding changes in theband gap and in the mobility are from Δ₀=1.1 eV to Δ₁ and from μ₀=14000cm²/Vs to μ₁. The transient time is given by the integration across baseregion 420 from 0 to W:

τ=∫dl/v[x(l)].  (4)

[0064] Note that the velocity is a function of x which in turn is afunction of the distance, l, across base region 420.

[0065] Literature data on the band gap and mobility for x=0-0.2 can beclosely approximated by the following linear relations:

Δ=Δ₀−αx, μ=μ₀−βx,  (5)

[0066] respectively, with α=1 eV and β=3250 cm²/Vs. For Profile A, theelectric field is constant: E=αx1/W, mobility is μ=μ₀−(βx₁/W) 1, andEquation 4 is reduced to

τ=(W/ax₁)∫dl/(μ₀−β*l )  (6)

[0067] with β*=βx₁/W. Integrating Equation 6, the transient time ofelectron carriers through E-B spacer 435 for Profile A becomes:

τ_(A)=(w²/αβX₁ ²) ln(μ₀/μ₁).  (7)

[0068] For Profile B, the velocity is constant: v=v₀, and therefore

τ_(B)=W/V₀.  (8)

[0069] From Eqs. 1 and 5, we have for the electric field:

E=αdx/dl.  (9)

[0070] Using Eqs. 3, 5 and 9, we obtain:

αdx/d1(μ₀−β_(x))=v₀.  (10)

[0071] After integration Equation 9 and substitution for the transienttime of electron carriers through E-B spacer 435 in Profile B becomes:

τ_(B)=W²/(αμ₀x¹−αβx₁ ² /2).  (11)

[0072] From Eqs. 9 and 11, we finally have:

τ_(A)/τ_(B)=[ (μ₀/β-x₁ ² /2)ln(μ₀ /μ₁) ]/x₁.  (12)

[0073] Substituting data for x₁=0.15, Equation 12 becomes: τ_(A)/τ_(B)=l.5.

[0074] According to the above analysis, the transient time for aconcentration profile according to Profile B is shorter than for thelinear profile of Profile A. For Profile B, the electric field issmaller near emitter region 410 and increases towards base region 420.This change in electric field may be attributed to the band gap changingmore slowly near emitter region 410 than it does near base region 420(see Equation 1). According to Equation 5, this in turn means that forProfile B, the Ge concentration changes more slowly near emitter region410 and faster near base region 420. In other words, moving across E-Bspacer 435, the Ge profile has a concave curvature (similar to line 220in FIG. 2) with the Ge concentration being smaller on the emitter sideof E-B spacer 435 and increasing to a maximum at the interface betweenE-B spacer 435 and the base region.

[0075] In the above example, a NPN HBT transistor was described having aSi_(1-x)Ge_(x) base region with an E-B spacer and a B-C spacer. Thespecific example described the E-B spacer. It is to be appreciated thatsimilar beneficial results may be obtained with a similarly optimizedB-C spacer, as well as an optimized base region. It is also to beappreciated that hole mobility generally increases with increasing Geconcentration (a behavior opposite to that of the electron mobility).Therefore, in the case of a PNP HBT, a convex profile (similar to line210 of FIG. 2) of Ge concentration in a spacer will generally result inthe shortest transient time and the highest cut-off frequency of PNPHBTs.

[0076] In the preceding detailed description, the invention is describedwith reference to specific embodiments thereof. It will, however, beevident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

What is claimed is:
 1. A method comprising: controlling the flow rate ofa species according to a determined concentration profile of a filmcomprising the species introduced on a substrate; and introducing a filmon a substrate, the film comprising the species at a first concentrationat a first point in the film and a second concentration different thanthe first concentration at a second point in the film.
 2. The method ofclaim 1, wherein determining the concentration profile comprises:determining a concentration of the species introduced on a substrate fora first plurality of flow rates; determining a growth rate of thespecies grown on a substrate for a second plurality of flow rates; anddetermining a concentration profile of the species for a unit of time.3. The method of claim 1, wherein the introduced film comprises athickness, the method further comprising: controlling the flow rate tointroduce the film at a graded concentration of the species throughoutthe thickness of the film.
 4. The method of claim 3, wherein the flowrate is controlled so that the graded concentration of the speciescomprises a linear gradient.
 5. The method of claim 1, whereincontrolling the flow rate comprises controlling the mass flow rate ofthe species.
 6. The method of claim 1, wherein the introduction of thefilm on a substrate comprises introducing the species and growing thefilm on the substrate.
 7. A machine readable medium comprisingexecutable program instructions that when executed cause a digitalprocessing system to perform a method comprising: controlling the flowrate of a species according to a determined concentration profile of afilm comprising the species introduced on a substrate, the filmcomprising the species at a first concentration at a first point in thefilm and a second concentration different than the first concentrationat a second point in the film.
 8. The medium of claim 7, whereindetermining the concentration profile comprises: determining theconcentration profile of the species for a unit of time according todata stored in a memory comprising: determining a concentration of thespecies introduced on a substrate for a first plurality of flow rates;determining a growth rate of the species grown on a substrate for asecond plurality of flow rates.
 9. The medium of claim 7, wherein theinstructions of the medium further comprise: controlling the flow rateto introduce a film at a graded concentration of the species throughouta thickness of the film.
 10. The medium of claim 9, wherein the flowrate is controlled so that the graded concentration of the speciescomprises a linear gradient.
 11. The medium of claim 7, whereincontrolling the flow rate comprises controlling the mass flow rate ofthe species.
 12. The medium of claim 7, wherein the introduction of thefilm on a substrate comprises introducing the species and growing thefilm on the substrate.
 13. A system for growing a film on a substratecomprising: a chamber; a species source comprising a species, thespecies source coupled to the chamber to introduce the species into thechamber; a mass flow meter coupled to the species source; and aprocessor coupled to the species source comprising a machine readablemedium comprising executable program instructions that when executedcause the processor to perform a method comprising: controlling theintroduction of the species into the chamber according to a determinedconcentration gradient of a film comprising the species introduced on asubstrate to introduce a film on a substrate, the film comprising thespecies at a first concentration at a first point in the film and asecond concentration different than the first concentration at a secondpoint in the film.
 14. The system of claim 13, wherein the mediumfurther comprises instructions that when executed cause the processor toperform a method comprising: determining the concentration profile ofthe species for a unit of time according to data comprising: determininga concentration of the species introduced on a substrate for a firstplurality of flow rates; determining a growth rate of the species grownon a substrate for a second plurality of flow rates.
 15. The system ofclaim 14, wherein the data relating to the concentration of the speciesand the growth rate are stored in the processor.
 16. The medium of claim14, wherein the introduced film comprises a thickness, the instructionsof the medium further comprise: controlling the flow rate to introduce afilm at a graded concentration of the species throughout a thickness ofthe film.
 17. The medium of claim 16, wherein the flow rate iscontrolled so that the graded concentration of the species comprises alinear gradient.
 18. A bipolar transistor comprising: a collector layerof a first conductivity type; a base layer of a second conductivity typeforming a first junction with said collector layer; and an emitter layerof the first conductivity type forming a second junction with said baselayer; an electrode configured to direct carriers through the emitterlayer to the base layer and into the collector layer, wherein at leastone of the first junction and the seconds junction is between differentsemiconductor materials to form at least one heterojunction, wherein theheterojunction has a concentration profile of a semiconductor materialsuch that an electric field changes in an opposite way to that of amobility.
 19. The bipolar transistor of claim 18, wherein the base layercomprises Si_(1-x)Ge_(x), where x is less than or equal to
 1. 20. Thebipolar transistor of claim 19, wherein the conductivity type of thebase layer is defined by a first dopant concentration and wherein thebase layer comprises a first spacer region between the emitter layer andthe base layer and a second spacer region between the base layer and thecollector layer, each spacer defined by a dopant concentration less thanthe first dopant concentration.
 21. The bipolar transistor of claim 20,wherein the concentration profile of the semiconductor material of theheterojunction comprises the concentration profile of at least one ofthe first spacer region and the second spacer region.
 22. The bipolartransistor of claim 21, wherein the conductivity type of the collectorlayer is N-type, the conductivity type of the base layer is P-type andthe conductivity type of the emitter type is N-type.